For example, FIG. 7 shows a known application apparatus comprising a die 102, which serves as a coating liquid supplying means, to supply a coating liquid 103 onto the upper face of a traveling web 101, and a freely rotatable metering bar 105, which serves as an application bar, provided downstream from the die 102 and supported by four rollers 104 (a pair at each side, i.e., each end in the width direction, totaling four) to meter or smooth the coating thickness of the coating liquid supplied onto the upper face of the web 101.
With such a conventional apparatus, however, feeding streaks 106 of the coating liquid or bubble streaks 107 taking in air from outside can be formed as shown in FIG. 8. Such feeding streaks 106 or bubble streaks 107 are generally regarded as coating defects. These coating defects are considered to result from uneven supply of the coating liquid 103 from the die 102, or result, as shown in FIG. 9, from air getting between the coating liquid 103 and the web 101 at the position indicated by the arrow A between the coating liquid 103 supplied and the web 101 if air is not removed properly by the metering bar 105.
With the aim of eliminating the defects, Patent Document 1 has disclosed an application method that assumes use of a relatively low-viscosity coating liquid. In the application method, a coating liquid is applied over a face of a web by supplying the coating liquid onto an application bar from a die provided near the application bar to form a bead at a contact portion of the application bar and the web.
In response to a request to develop a technique for applying a high-viscosity coating liquid having a viscosity of not less than 0.1 Pa·s (100 cp), the present inventors attempted use of an application method as described in Patent Document 1. It was found, however, that the application method cannot prevent formation of feeding streaks in a coating applied over a face of a web.
It has been also found that when using a high-viscosity coating liquid having a viscosity not less than 0.1 Pa·s (100 cp), furthermore, air is taken in between a coating liquid 103 and an application bar 105 at position indicated by the arrow B between the coating liquid 103 supplied onto the application bar 105 and application bar 105 as shown in FIG. 10, and the coating liquid 103 containing air flows into a bead 100, resulting in bubbles being caught under the application bar 105 to form bubble streaks in a coating film on a web.
Moreover, if uneven supply in a web's width direction takes place when supplying the coating liquid from a die or nozzle, such unevenness would not be eliminated completely by the application bar, resulting in an uneven thickness distribution of the coating liquid applied. To preventing this, the slit gap of the die may be decreased as a means of feeding the coating liquid uniformly in the web's width direction. But, cleaning of the lip portion would be difficult to perform if the slip gap is small. If a width of the die (the width is in the width direction of the web) is large and a coating liquid feeding pressure is high, the lip gap would be increased by a high internal pressure, making it impossible to maintaining a uniform gap in the width direction.
By the way, a slide coater that appears to be similar to a preferred embodiment of the invention is disclosed in Non-Patent Document 1.
Patent Document 1: JP 2805177 B
Non-Patent Document 1: “Theory and Explanation of Die Coater; Coating—Learning for Future from Equipment Technology in the Past and Present—”, Yoshinobu Katagiri, Japan, Converting Technical Institute Co., Ltd., Mar. 25, 2002, p. 399